The present invention is related to a passive depressurization system, and more particularly to a system for passively depressurizing the reactor coolant system of a pressurized water nuclear reactor during the process of bringing the reactor to a cold shutdown state.
A typical pressurized water reactor includes, inter alia, a reactor vessel which contains nuclear fuel, a coolant heated by the nuclear fuel, and means for monitoring and controlling the nuclear reaction. The coolant is frequently water and, for the sake of convenience, the coolant will be referred to as "water" hereafter although it will be understood that other coolants could be used. The water heated in the reactor vessel is conveyed from and returned to the vessel by a reactor coolant system which includes one or more reactor coolant loops, each loop including a hot leg conveying hot water from the reactor vessel to a steam generator and a cold leg returning the water to the reactor vessel. The steam generator is a heat exchanger which transfers heat from the reactor coolant system to water from a source isolated from the reactor coolant system; the resulting steam is conveyed to a turbine to generate electricity.
During normal operation of the reactor, the water within the reactor vessel and the loops of the reactor coolant system is maintained at a high pressure in order to keep the water from boiling. A pressurized is provided to regulate the pressure. Since the reactor vessel and all of the loops of the reactor coolant system are hydraulically connected, only one pressurizer is needed to regulate the pressure within the entire system. The pressurizer includes a vessel which is filled with water and steam at equilibrium conditions and which is hydraulically connected to the hot leg of one of the reactor coolant system loops. The pressure within the system is sensed by transducers within the pressurizer. If the pressure is too low, immersion heating elements within the pressurizer are turned on in order to generate additional steam within the pressurizer and thereby raise the pressure, this increased pressure being communicated to the reactor vessel and the reactor coolant loops connected thereto. On the other hand if the pressure is too high, relatively cool water from the cold leg of one (or more) of the reactor coolant system loops is sprayed into the pressurizer in order to condense some of the steam therein and thereby reduce the system pressure. As a safety feature, a valve system is provided to vent the pressurizer to a pressure relief tank in order to ensure that the pressure within the system does not become excessive.
Due to the radioactive materials employed in a nuclear reactor, extreme care must be taken to avoid a malfunction which might release fission products into the environment. Merely designing the system to withstand the rigors of normal operation is not sufficient, since components may malfunction and anomalous events, such as seismic shocks, may occur. In short, in the interest of public safety the reactor must be designed so as to avoid, under all credible conditions, any substantial risk that harmful amounts of radioactive materials may be released into the environment. The public safety is, of course, a joint concern of Government and those in the nuclear industry.
In the United States, the Government has determined that a pressurized water reactor should be designed so that it can be brought to a cold shutdown condition within 36 hours, under any realistic accident condition and using only safety related equipment. This basically means that it must be possible to reduce the temperature and pressure of the water to less than 200.degree. F. and 400 PSIG (2.76.times.10.sup.7 dynes/cm.sup.2, gauge pressure) within 36 hours, even in the event that electrical power is lost or some other failure occurs, using highly reliable components that have been certified to operate under certain adverse conditions (rather than control grade components, which are suitable for use under normal conditions but which have not been qualified under various standards).
After the nuclear reaction has been stopped, the cold shutdown can be achieved in two stages. The first stage, which will be discussed in more detail below, is to cool and depressurize the reactor coolant system to an intermediate level, such as 350.degree. F. (177.degree. C.) and 400 PSIG (2.76.times.10.sup.7 dynes/cm.sup.2, gauge pressure). At this point the second stage is begun, and is conducted by a residual heat removal system. A typical residual heat removal system includes two heat removal units, each heat removal unit including a heat exchanger, valving, and a pump to convey water through the valving and heat exchanger from a hot leg to at least two cold legs. It is of course redundant to use two heat removal units, each connected to at least two cold legs, but this redundancy provides a safety feature which permits a cold shutdown to be achieved even in the event that one of the heat removal units fails. The components in the heat removal units are sufficiently robust to withstand the intermediate pressure and temperature that is achieved upon completion of the first step.
The first stage of the cold shutdown procedure typically employs the steam generators to reduce the heat in the reactor coolant system. To reduce the pressure, steam is typically vented from the pressurizer to the pressure relief tank. This venting of steam to the pressure relief tank has several disadvantages. First, the water condensed from the steam must subsequently be decontaminated. This is expensive and may, moreover, delay the time when operation of the reactor can resume. Furthermore the steam is preferably vented in several steps by opening and closing motor operated valves. Each operation of the valve increases the possibility of mechanical failure, even assuming that electrical power is available.